The NIST Chemistry WebBook provides users with easy access to
chemical and physical property data for chemical species through
the internet. The data provided in the site are from collections
maintained by the NIST Standard Reference Data Program and outside
contributors. Data in the WebBook system are organized by chemical
species. The WebBook system allows users to search for chemical
species by various means. Once the desired species has been
identified, the system will display data for the species.

If any search finds more than one chemical species that match
the search criteria, the matching species will be displayed on a
selection page. At a minimum this page displays the name and
chemical formula of each species found. If there are only a small
number of species, the structure of the species will also be displayed
if it is available. A user can display data on a particular
species by clicking on its name. Data for other species found in
the search can be viewed by returning to this page and clicking
on their names.

Most of the search types discussed above share several common
features. These features include the ability to select the
units to be used, the ability to select the types of data to
be displayed and a common format for displaying the results
of a search where more than one species match the search
criterion.

Searches involving phyical quantities may be restricted based
on chemical formula. This feature allows one to restrict a
search to a class of compunds. For example, this feature
could be used to restrict a search to hydrocarbons only.
These restrictions work just like the
Formula Search, except they
do not allow ions to be excluded.

Each of the direct search types allows the user to select
the units to be used for displaying data. The user may choose
between SI units and calorie-based units. Choosing the latter
option will result in energy and pressure values displayed in
calories and atmospheres rather than Joules and bars.

Most of the search types allow the user to select
the type of data to be displayed. The user may select any
number of the following options:

If the search succeeds, data corresponding to the selected
options will be displayed. If none of the data types are selected,
only general information about the species will be displayed.

For all of the direct search types except the registry number and
name searches, selecting one or more of the data type options
will affect the search criteria. If a data type is selected, a
species will be considered a match only if it contains one or more
of the data types selected. For example, consider a chemical formula
search with both the "Gas phase thermochemistry data" and "Mass spectrum"
data types selected. This search will match all species with the
given formula that have either gas phase thermochemistry data
or a mass spectrum. Species that do not contain any of the selected
data types will be skipped. If no data types are selected, all
species matching the search criteria will be found.

The formula search routines will find all of the species
in the database with the requested chemical formula. The
formula may be specified in any order. Legal characters in
formulas include upper and lower case letters, numbers,
question marks, asterisks, and parentheses. Formulas may
be specified in a semi-structural manner, but any implied
structure or connectivity in the formula will be ignored.
For example, (CH3)(CH2)4(CH3) will find all species
in the database with the formula, C6H14,
not just n-hexane.

When specifying the formula, the proper case should be
used where necessary to properly identify atomic symbols.
A search for a formula of no, NO,
or nO will result in nitric oxide being found,
while a search for No will result in nobelium
being found.

A range of formulas may be searched for by using a wildcard
character for an element amount. If a wildcard is used,
parentheses cannot be used in the formula. There are two
wildcards, the question mark and the asterisk:

?

The question mark matches one or more atoms of the
element that proceeds it. For example, CH? will
find all species with one carbon atom and one or more
hydrogen atoms.

*

The asterisk matches any number of atoms of the element that
proceeds it. Its behavior is exactly the same as that of the
question mark except that it will match a species with no atoms
of the specified element.

A wildcard specification may be combined with a normal
specification for an element. Thus C2C*H4 will search
for species with two or more carbon atoms and exactly four
hydrogen atoms.

The formula search may be modified by four check boxes.
The effect of each check box is described below:

Exactly match the specified isotopes

When checked, this option indicates that the search
will match only the isotopes specified in the search
formula. When not checked, other isotopes will be
matched. If one searches for H2 with this
option checked, the search will find just the hydrogen
molecule, H2. If this option is not
checked, the search will find additional species,
including D2 and T2.

Allow elements not specified in formula

When checked, this option indicates that species
containing elements not specified in the formula
will also be matched. For example, a search for
Te2 will find all of the species which
contain exactly two tellurium atoms and any number of
other atoms. Checking this option alone is equivalent
to the "partial formula search" previously available
from this site.

This option should be used with care as it could greatly increase
the number of matches found by a search. The number of matches in
a search can be reduced by specifying one or more atoms that
should not be present in the desired species. This is done by
placing a zero after the atomic symbol for the species in the
formula. For example, a search on CH2N0O0 will
find species that have one carbon, two hydrogens, no oxygen, and
no nitrogen, but could contain any number of other atoms.

Allow more atoms of elements in formula than specified

This option indicates that species containg more
atoms than specified of a given element will be matched.
For example, a search for Te with only
this option checked will find all tellurium, ditellurium,
tritellurium, etc.

Exclude ions from the search

When this option is checked, the search will not
match ions unless a charge is explicitly indicted in
the formula.

The chemical name search option looks up chemical species by
their name. The search is not case sensitive, so it is
not necessary to use the correct case. Spaces are also
ignored by the search algorithm, so both
hydrogenfluoride and
Hydrogen Fluoride
will find HF. The database includes some common trade names
and acronyms for chemical species. Because of this, it
is possible that one name may be used by more than one
species in the database. In this case, a selection
page containing all of the matching species will be
displayed.

The name search can also match patterns. Patterns are
name strings which contain the asterisk (*) character.
Each asterisk in the pattern may match zero or more
characters in the name. Thus a search on *heptanone
will look for species with a name that ends with
"heptanone". A search on *heptanoic acid
will find species with names that contain the text
"heptanoic acid" somewhere in the name. When using patterns
it is important to remember that the set of names used
by the site is incomplete, so searching on a particular
pattern may not match all of the desired species (since
the database may not have the name which matches the
pattern for some species).

If a species with a matching name is not found, the given
string is assumed to be a chemical formula and any matching
species are displayed. If a matching formula is not found,
the given string will be assumed to be a CAS registry number
and any matching species will be displayed.

If a name search fails, it is recommended that the user
attempt a formula search to find the compound, since the
species may be in the database under another name.

The ionization energy search searches for species with a given
ionization energy. Like the molecular weight search, either a
single value or a comma-separated range can be used for the
search. In all cases the values must be in electron volts (eV).
If a single value is entered, the system will search for species
with an ionization energy within 0.05 eV of the specified value.

The search only covers those species which have an ionization
energy value that has been evaluated and assigned for search
purposes. Currently this amounts to only 40 percent of the
species with ionization energy data. This means that this search
may miss some species with experimental IE determinations within
the desired value range.

The electron affinity search looks for a species with
a given electron affinity. The search is carried out
on either a single value or a comma separated range
in electron volts (eV). If a single value is entered,
the system will search for values within 0.05 eV of
the given value.

Where possible, electron affinity data is listed under
the neutral species. In some cases, the corresponding
neutral species does not exist or is not included in
our database. For these cases, the electron affinity
data will be listed under the anion. The electron
affinity search will find whichever species the
electron affinity value is listed under.

The proton affinity search covers only those compounds
in the database for which proton affinity values have
been defined. Searches for a single value will look
for proton affinities within 5.0 kJ/mol (or 1.2 kcal/mol)
of the specified value. Unlike some of the other value-based
searches, the units for the specified value or range
may be selected via radio-style buttons. The units used
for the search value will also be used for the display
of data.

The acidity search looks for species that have a
given acidity value. Acidity is the Gibbs free
energy of the reaction in which an acid dissociates
into an anion and a hydrogen ion. Acidity values
or comma separated ranges may be specified in
kJ/mol or kcal/mol. If a single value is entered,
the system will search for values within 5.0 kJ/mol
(1.2 kcal/mol) of the specified value.

Unlike other ion energetics related values, acidity
is listed as a Gibbs free energy of reaction value
in the Reaction Thermochemistry Data
section. The search will provide a link to the anion;
the acidity value(s) can be found by looking for Gibbs
free energy values under the appropriate reaction in
the reaction thermochemistry data section.

The appearance energy product search looks for appearance energy
(AE) reactions that result in a given positive ion as a product.
Since the actual structure of the product is often not known,
this search is based only on the chemical formula for the product.
This search will always look for positive ions, so it is not
necessary to include a plus sign (+) in the formula.

If there are data with the given AE product, all the data will be
displayed in the form of a pseudo-species that represents all
of the positive ions with a given formula. This pseudo-species
will only contain AE data.

There are two options available for searching the vibrational
and electronic spectra available in the site. Searches may
be conducted on vibrational energy values and electronic
(band origin) energy levels. Please note that these searches
do not search the gas phase infrared spectra available
from this site.

The vibrational energy search looks for species with
a specified vibrational energy. A single value or a
comma separated range may be used for the search. In
all cases the energy value should be in wavenumbers
(cm-1). If a single value is given, the
system will search for values within 5.0 cm-1
of the given value.

The electronic energy level search finds species with
electronic transitions with a given band origin. A
single value or a comma separated range may be used
for the search. In all cases the energy value should
be in cm-1. If a single value is given, the
system will search for values within 5.0 cm-1
of the given value.

There are several options available for structure based
searching. These searches can require substantially
more computational resources than the other searches
available through the site, so they will often take longer
to process. All of the searches are limited to the species
that have structures in the database, so it may be wise to
use them in concert with other search methods. The three
types of structure based searches are the applet based search,
the file based search, and the structure class search.

The applet based search provides an applet that allows
users to draw structures they wish to search for. The
applet requires a Java capable browser to run. When the
structure has been drawn the user may search for matching
structures or superstructures of the structure drawn. When
searching for superstructures of a given structure (substructure
search) additional groups may be attached to empty valences.
The number of matching substructures may be controlled
by specifying the numbers of hydrogen atoms attached to
an atom. For example, a substructure search on C-C
will match any species with a carbon–carbon single bond
while a substructure search on H3C-CH2
will match species with an ethyl group.

The file based structure search is similar to the applet
based structure, except it requires a user supplied structure
to be uploaded to the web server. Because of this, the search
requires a browser that can upload a file to a web server.
Submitted structures should be in MDL mol file format.
The choice of file format does not reflect an
endorsement by NIST of any company's product or file format,
nor does it imply that any file format or product is more
suitable for this task at hand.

The structure class search allows users to find
structures based on characteristics of the structure. These
characteristics are selected through a form interface. The
following characteristics can be specified through this search:

The number of atoms (excluding hydrogen) in the structure.

The number of bonds in the structure.

The number of rings in the structure.

The chemical formula.

The number of occurrences of a specified functional
group in the structure.

The structures in this site have been pre-classified to speed
up this search. It is important to note that functional groups
used in this search are identified independent of each other.
Because of this a carboxylic acid group is also considered
to be an OH group and C=O group. For this
reason, a user may wish to exclude species containing carboxylic
acid groups when searching for species containing an
OH group.

The molecular weight search finds species matching a
specific molecular weight value or within a comma separated
range. If a single value is specified, the search will find
any species with an atomic weight within 0.5 units
of the specified value.

The molecular weight search uses atomic weights based
on the most abundant isotope for the element. This is in
contrast to more commonly used atomic weight values based
on an average of isotope weights. The molecular weights
displayed on the species data pages are based on isotope
average atomic weights. Because of this, the weights used
in the search may differ from those displayed in the
species data pages.

The reaction search allows one to search directly for
reaction data without first searching for individual
species. Reactions are selected by entering one of
the following for each reactant or product of interest:

chemical formula

chemical name

CAS registry number

Options are supplied which allow the user to select reactions
in either direction, allow reactants and products in addition
to those specified, exclude reactions with ions, and select a
specific reaction type. The options for matching reactions in
either direction and allowing additional reactants and products
are pre-selected since it is envisioned that they will be
desired by a majority of users.

The author search facility allows searches of the
reference database used for this site by author names.
Up to three author names may be specified. The year
of publication or a range of years may be specified
to narrow the search. Author names should be specified
as a last name followed by initials (e.g.,
Doe, J.A.). Since there
are references in the database for which currently
have no data abstracted, this search can find references
for which do not otherwise show up in our database.

Indirect searches are searches that are carried out
by clicking on a highlighted region of text. Unlike
direct searches, these searches cannot initially be
used to find data. They can only be used to find data
that are related to data which have already been displayed.

A reaction species search can be carried out by clicking on
the species of interest in a chemical reaction display. The
search will display data for the species selected. This feature
will not work for the species currently being displayed or for any
species that do not have data in the database.

In many cases it may be more convenient to use the
reaction search facility
to search for specific reactions.

An indirect author search is carried out by clicking
on an author's name in a reference citation. This search
will look for all of the reference citations in the
database that contain the author's name. It should
be noted that this search is limited to the WebBook
database and, therefore, should not be confused
with a complete literature search.

A source search can be carried out by clicking on
the expression, [all data], at the end of
a reference citation. Such a search will display all
of the species in the database that contain data
from the source. If only one species contains data
from the source, that species will be displayed.

Several types of data can be displayed with a graphical
display. In some cases a Java applet is used to
generate the display. In these cases, users
will not be able to run the applet if they do not
meet the following conditions:

The browser is Java capable.

The browser has Java enabled.

The browser is not behind a firewall that
blocks Java applets.

The system that displays infrared mass and UV/Vis spectra
has been designed to display static images of the
spectra in cases where the applet cannot be loaded.

Mass and IR spectra may be displayed as an inline image or
as an applet. Inline images are only used on browsers that
do not support applets or have applet support turned off.

When spectra are displayed by the applet, the user may zoom
the image by selecting a region to be zoomed with the mouse.
Single clicking the mouse will un-zoom the spectrum. For IR
spectra, the direction of the x-axis can be reversed by toggling
a button on the applet. The applet will track the coordinates
of the mouse in the upper right-hand corner of the display.
This feature can be toggled off and on by clicking within
the applet and pressing the C
(for coordinate) key on the keyboard.

Spectra displayed as images can also be zoomed, but the options
for zooming are more restrictive than those of the spectra
display applet. To zoom spectra images, the following steps
must be carried out:

The desired x-axis range must be entered. The range should
be entered as two values separated by a comma. The first value
will be used for the left endpoint of the x-axis and the
second will be used as the right endpoint.

If desired, the "automatic Y scaling" button can
be checked. If the button is checked, the y-axis will be
adjusted so that it is appropriate for the range of data
displayed. If the button is not checked, the original
y-axis range is used.

The "zoom" button must be pressed. This will load a new
page with the desired spectra image.

Fluid property data may be optionally displayed in an applet
which provides the ability to generate X-Y plots or display
data in tabular form.

After the applet has loaded the data from the server,
X-Y plots may be displayed by selecting the desired data
types from selection boxes for each axis. The region
displayed may be enlarged by selecting a region of the
plot with the mouse. Data are color coded by state
as noted below:

Color

State

Blue

liquid

Red

gas

Green

supercritical

Magenta

saturation curve

The applet has a button in the upper left hand corner that
will toggle the display of a table containing the raw
physical property data. The data displayed in the table
can be set by selection boxes which are located above
each column in the table.

Some data in the WebBook come in the form of
function parameters that describe the behavior
of a physical property over a given range. For
many of these data types it is possible to select
a link that will load an applet which will display
the curve generated by the function. In the case
where multiple determinations of the parameters
are available, all of the resulting curves will
be plotted together.

As with the other plot types the function plot may be
zoomed by selecting a region on the plot. Depending
on the type of data plotted, the data may be initially
plotted using log-log, semi-log, or normal axes.
Information on changing the plot format can be found
by pressing the "help" button on the applet. Data
from different sources will be plotted in different
colors; a legend box is provided to identify the
sources.

For a chemical species there are several types of
data which can be obtained from the WebBook system.
However, all but a few of the chemical species
in the database contain only a subset of the
possible data types.

Data are broken up into types with each type separated
by a horizontal line across the web page. General
information about the species is always displayed at the
top of the page. Reference citations, if present, and
notes are displayed at the bottom of the page.
If requested, groups of data are displayed between
the general data section and the reference citations.

Parameters for enthalpy of vaporization equation (form of
equation given with parameters).

Entropy of vaporization at standard conditions
(ΔvapS°).

Entropy of vaporization at a given temperature
(ΔvapS).

Enthalpy of fusion at standard conditions
(ΔfusH°).

Enthalpy of fusion at a given temperature
(ΔfusH).

Entropy of fusion at standard conditions
(ΔfusS°).

Entropy of fusion at a given temperature
(ΔfusS).

Enthalpy of sublimation at standard conditions
(ΔsubH°).

Enthalpy of sublimation at a given temperature
(ΔsubH).

Entropy of sublimation at standard conditions
(ΔsubS°).

Entropy of sublimation at a given temperature
(ΔsubS).

Antoine equation (vapor pressure) parameters.

Phase transition temperature, (Ttrs).

Enthalpy of phase transition at a given temperature
(ΔtrsH).

Entropy of phase transition at a given temperature
(ΔtrsS).

For phase transitions other than vaporization, freezing, and
sublimation, the phases involved in the transition are listed
in the table with the data. Unless otherwise stated, all
data are for species at standard pressure.

The heat of combustion data (if available) are
grouped together in a single table at the start
of the section. Data for specific reactions
are provided in tables following a schematic
representation of the reaction. These data may
include the following values:

Enthalpy of reaction.
(ΔrH°).

Entropy of reaction.
(ΔrS°).

Gibbs free energy of reaction.
(ΔrG°).

Unless stated otherwise, all data are for reactions
occurring at 298 K. The enthalpy of reaction data
are grouped by reaction. Please note that for anions,
some of the reactions presented correspond to heat of
acidity or heat clustering values.

Positive ion data are generally listed under the
corresponding neutral species. With the exception
of electron affinity data, negative ion data are
provided under an entry for the anion. This means
that if you want positive ion data, you must search
for the neutral species, and if you want negative
ion data you must search for the anion. This
potentially confusing situation is a legacy of
the format in which the data were collected and
the fact that many of the anions in the database
do not have corresponding neutral species. In those
cases where the corresponding neutral is known,
any electron affinity data will be listed under
the neutral species, since the electron affinity
may be considered a property of the neutral species.

In addition to the data described above, a link
to search for appearance potential reactions with
a product with the same formula as the current
species may be displayed. Clicking on this link
is equivalent to conducting an
Appearance Energy Product
Search using the chemical formula of the current
species.

Data for ion clustering reactions is available in this
section. This data is also available in the
Reaction Thermochemistry
section, but this section limits reactions to only
ion clustering reactions. This makes it easier to find
clustering reactions. For example, a search on a neutral
species with ion clustring data specified, will find
all matching species which are ligands in at least one
ion clustring reaction.

Where available, gas phase spectra from the
NIST/EPA Gas-Phase Infrared Database
may be displayed as a static image or through
an applet. If available, supplemental data
will be displayed below the spectrum.

Mass spectra are provided from the NIST Mass Spectrometry
Data Center. Supplementary data, such as the source of the
spectrum, instrument type, instrument parameters, and the
EPA MS number are displayed below the spectrum.

Vibrational and electronic spectra are provided in tabular
format. Where appropriate, spectra are grouped by state,
with the states in order of decreasing energy. Tables containing
vibrational spectra contain the following data:

Vibrational symmetry

Vibration number

Approximate mode type

Wavenumber

Medium used in the measurement

References

Some of the vibrational data tables may have more
data depending on the source of the data.

Constants of diatomic molecules are provided in tabular
form. These data contain a large number of notes and comments.
Some comments are embedded directly in the table while
others are provided in a list of notes which follows the
table. The following quantities are provided:

State

Energy, Te

ωe
(vibrational constant)

ωexe
(vibrational constant)

ωeye
(vibrational constant)

Be
(rotational constant)

αe
(rotational constant)

rotation-vibration interaction constant,
γe

De
(rotational constant)

βe

Internuclear distance in Å, re

Transition

ν00

Most of the constants are derived from the Dunham
series expansion:

TνJ =

∑

Ylm(ν + ½)l
Jm(J + 1)m

l,m

which results in the following:

G(ν)

=

ωe
(ν + ½) −
ωexe
(ν + ½)2 +
ωeye
(ν + ½)3 +
ωeze
(ν + ½)4 ...

Fν(J)

=

BνJ(J + 1) −
DνJ2(J + 1)2 +
HνJ3(J + 1)3 ...

Bν

=

Be −
αe(ν + ½) +
γe(ν + ½)2 + ...

Dν

=

De −
βe(ν + ½) + ...

When available, references are supplied on a line
directly below the values they refer to. A directional
arrow ()
is placed at the start of the reference line to indicate that they
apply to the data on the line above.

Gas chromatographic retention data consists of Kovats and Lee
retention indices for a wide variery of compounds. Data are
oganized based on column type (non-polar, polar, or other) and
heating program (isothermal, simple ramp, or complex program).
The initial set of data includes only data from non-polar (or
nearly non-polar columns). It is anticpated that data from
other column types will be added in the future.

A special facility provides fluid properties over user-specified
ranges for a group of pure fluids. This facility can provide
data over the following range types:

Isobars

Isotherms

Isochors

Saturation curve — pressure increments

Saturation curve — temperature increments

The following types of data are available:

Temperature

Pressure

Density

Specific volume

Heat capacity at constant pressure (Cp)

Heat capacity at constant volume (Cv)

Enthalpy

Internal energy

Entropy

Speed of sound

Joule-Thompson coefficient

Thermal conductivity

Viscosity

Surface tension (along saturation curve only)

Please note that for data along the saturation curve,
all properties except temperature and pressure will
be provided for both the liquid (l) and vapor (v)
phases.

In order to receive data from this facility, a series
of web pages must be completed that provide selections
for units, range type, range parameters, and display
type. There are currently two types of displays available:
an applet that displays X-Y plots of selected data and
a data table with user selectable axes and a HTML table
that contains the raw data.

Citations for the literature referenced in a page are listed
in a section at the end of the page. The citations are listed
in the order in which they are referenced. The reference to
the citation used in the data tables is given in bold print
above each citation.